Stress-free crimp joint for plastic-to-metal interfaces

ABSTRACT

A stress-free crimp joint for plastic-to-metal interfaces for prevention against stress cracking of the plastic member. The plastic member has a dovetail flange that is placed in a channel in the metal member. The lip of the metal member that surrounds the flange is circumscribed with a groove. When the lip is crimped over the plastic flange, the groove ensures that the lip bends along an optimum, line, defined by the groove, and that none of the plastic is pinched after the crimping action. The angle and magnitude of the crimp are carefully controlled so that after the crimping tool is removed the elastic springback of the lip is just sufficient to relieve any load on the plastic.

United States Patent Murray Ressler; George K. Lucey, Jr., both of Silver Spring,

[72] Inventors Md. [21] Appl. No, 21,734 [22] Filed Mar. 23, 1970 145] Patented July 20, 1971 [7 3] Assignee The United States of America as represented by the Secretary of the Army [54] STRESS-FREE CRIMP JOINT FOR PLASTIC-TO- [56] References Cited UNITED STATES PATENTS 638,564 12/1899 Davies 287/91 1.008.415 11/1961 Foley. l02/70.2P

ABSTRACT: A stress-free crimp joint for plastic-to-metal interfaces for prevention against stress cracking of the plastic member. The plastic member has a dovetail flange that is placed in a channel in the metal member. The lip of the metal member that surrounds the flange is circumscribed with a groove. When the lip is crimped over the plastic flange, the groove ensures that the lip bends along an optimum, line, defined by the groove, and that none of the plastic is pinched after the crimping action. The angle and magnitude of the crimp are carefully controlled so that after the crimping tool is removed the elastic springback of the lip is just sufficient to relieve any load on the plastic.

STRESS-FREE CRIMP JOINT FOR PLASTIC-TO-ME'IAL INTERFACES RIGHTS OF THE GOVERNMENT The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION This invention relates generally to artillery fuze radomes and more particularly to the crimp joint between a plastic fuze radome and its associated metal fuze body.

Artillery fuze radomes are ordinarily made from thermoplastics, which have desirable electrical and mechanical properties, are low cost, and are amenable to mass production. The major disadvantage of thermoplastics is their inherent tendency to stress crack. This means that when the plastic is placed in a state of tensile or shear stress, it will eventually crack even though the stress level may be considerably below the rated strength of the material. A common method in artillery fuzing of joining the plastic radomes to the metal f uze body or housing is by crimping a portion of the metal body over a portion of the plastic nose cone. It has been found that this procedure places the plastic nose cones in a state of stress, thus exposing the finished product to the aforesaid stresscracking tendencies. Different plastics take varying lengths of time to stress crack, and a common military requirement is that the material survive storage for a period of years. If cracks do develop during storage, the mechanical integrity of the nose cone is lowered, the moisture resistance is destroyed, and the chances of early fuze functions is increased when the projectile is fired into rain, snow, or clouds.

It has been suggested that if the proper data were available in the literature, then the nose cone could be designed to avoid cracking for at least 20 years. However, the data are usually unavailable and the practice of designers has been to disregard the potential dangers of stress cracking. This danger is reduced somewhat by using glass fiber filled plastics which have recently become available, but the true sensitivity of these materials to stress cracking is not known. It appears, therefore, that the only valid means of avoiding stress cracking in military hardware that involves a metal-to-plastic joint is to use a stress-free attachment.

Accordingly, the primary object of the present invention is to provide means for joining a plastic nose cone to its metal housing in a stress-free manner and thus permanently avoid any stress-cracking tendency.

Another object of the present invention is to provide a stress-free joint between a plastic nose cone and its metal housing by means of a simple modification to known hardware.

SUMMARY OF THE INVENTION Briefly, in accordance with the invention, a stress-free crimp joint is provided to guard against stress cracking in artillery fuze radomes. The plastic radomes are crimped by the lip of a metal housing in such a manner as to prevent any of the plastic from being permanently deformed by the lip. A groove is located around the circumference of the lip to ensure the bending takes place at the proper position. If the angle and magnitude of the crimp are carefully controlled, a stress-free joint is assured. The freedom of the nose cone to rotate after the crimping operation relative to the metal body is the signal that the joint is indeed free from stresses. Various sealants may be used to prevent entrance of dirt and moisture through the joint.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the invention as well as other objects, aspects, uses, and advantages thereof will clearly appear from the following description and from the accompanying drawings, in which:

FIG. IA is a first illustration of the prior art of the technique of attaching a plastic nose cone to its metal housing shown in a cutaway side view;

FIG. 18 is a second illustration of the prior art;

FIG. 2 is a cutaway side view of a preferred embodiment of the present invention of a plastic nose cone and metal housing prior to joining;

FIG. 3 is an enlarged sectional view of portion A of FIG. 2 showing the plastic flange of the nose cone inserted in the metal housing just prior to crimping;

FIG. 4 illustrates the same essentials as FIG. 3 just after the crimping operation but before elastic springback has occurred;

FIG. 5 shows the final form of the crimp joint between the plastic flange and the metal housing; and

FIG. 6 is an overall partially cutaway view of the completed stress-free crimp joint between the plastic nose cone and the metal housing.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. IA shows a plastic nose cone 10 that has been molded onto its fuze body I2 and is representative of the prior art of nose cone attaching techniques. Inherent stresses were created at the joint I4 that led to cracking in the plastic by mere virtue of the method of attachment and the contact between nose cone 10 and body 12. Another example of the prior art is depicted in FIG. 18 which shows a plastic nose cone 16 that has a flange l7 fitted into a hollowed out receptacle in the body 18. After insertion, the lip 20 of body 18 was crimped over flange I7 to create the joint 22. It was found that the crimping action set up stresses in the nose cone by virtue of lip 20 permanently deforming the outer portion of flange 17 at joint 22. The residual stresses with this type of attachment can be very high, but even if they are low only implies that the cracks would take longer to appear and thus require the future repair of stockpiled fuzes, an expensive procedure.

In our FIG. 2 embodiment nose cone 24, which is essentially circular in cross section, has a dovetail flange 32 at its base which is designed to fit into a channel 84 in body 26. Body 26 has a lip 30 for crimping over flange 32. Lip 30 has a circumferential groove 28 at the optimum position for bending of lip 30 in the crimping operation.

FIGS. 3, 4 and 5 are enlarged views of portion A of FIG. 2 and illustrate sequentially the details of the crimping operation that results in the stress-free joint of the present invention. FIG. 3 shows flange 32 placed within body 26. Note the relative position of groove 28 with respect to the angle 29 of flange 32. Groove 28 ensures that lip 30 bends at the proper position, as shown in FIG. 4. The position of the bend 34 is important to ensure that none of the plastic of flange 32 below that point is pinched by the crimping action, thus providing adequate spacing 42 between body 26 and flange 32. The magnitude of the crimping force is carefully controlled to ensure that other areas of body 26 are not deformed. It is seen in FIG. 4 that the crimping force has resulted in a momentary deformation of the region 40 of flange 32 by lip 30. The angle of the crimping force, however, is judiciously selected in conjunction with the magnitude so that after the crimping tool is removed, the elastic springbaclt of lip 30 is just sufflcient to relieve any load on plastic flange 32, as illustrated by FIG. 5 which shows the final stress-free joint. An indication of the completion of a stress-free joint is that nose cone 24 is free to rotate by hand relative to body 26 after the operation. If the edge 36 of cone 24 is in substantial alignment with the edge 38 of body 26, this indicates that the crimping force was applied with the proper magnitude and angle, and that the requisite spacing 42 has been achieved. Various known sealants may be utilized to make the joint solid and weatherproof.

FIG. 6 shows the final stress-free attachment between nose cone 24 and body 26. It is evident that groove 28 encircles the entire body and that lip 30 can be crimped by a number of methods well known in the art, keeping the aforesaid requirements in mind. Wind tunnel experiments have additionally demonstrated that the present invention provides a greater degree of reliability with respect to separation between the nose cone and body during flight.

We claim as our invention: 1. A stress-free crimp joint for a plastic-to-metal interface, comprising a. a plastic member having a dovetail flange protruding from an open end thereof; b. a metal member complementary to said plastic member,

comprising l. a channel adapted to receive said dovetail flange;

2. an outer wall that is crimped under a predetermined pressure over said flange to prevent their separation;

3. a groove circumscribing said outer wall and located so as to allow elastic springback thereof and relieve any load on said flange to ensure that said plastic flange is not permanently stressed by said crimped outer wall whereby said plastic member is free to rotate relative to said metal member.

2. The invention according to claim I wherein said plastic member is an ordnance fuze radome.

3. The invention according to claim 2 wherein said metal member is an ordnance fuze body.

4. A method of fabricating a stress-free joint between a plastic radome and the metal body of an ordnance fuze, comprising the steps of:

a. providing a dovetail flange at the open end of said radome;

b. providing in the open end of said metal body a channel for receiving said flange;

c. providing on said metal body a metal lip adapted to be crimped over said flange;

d. providing on said lip a circumferential groove at the optimum position for bending of said lip during crimping without permanently applying stress to said flange by allowing elastic springback of said lip after crimping;

e. positioning said flange in said channel;

f. applying a predetermined crimping force to said lip, said force causing said lip to bend inward at said groove, said force being sufficient to cause the bent portion of said lip to apply stress to said flange during the continued application of said force but insufficient to cause said lip to apply stress to said flange after removal of said force. 

1. A stress-free crimp joint for a plastic-to-metal interface, comprising: a. a plastic member having a dovetail flange protruding from an open end thereof; b. a metal member complementary to said plastic member, comprising
 1. a channel adapted to receive said dovetail flange;
 2. an outer wall that is crimped under a predetermined pressure over said flange to prevent their separation;
 3. a groove circumscribing said outer wall and located so as to allow elastic springback thereof and relieve any load on said flange to ensure that said plastic flange is not permanently stressed by said crimped outer wall whereby said plastic member is free to rotate relative to said metal member.
 2. an outer wall that is crimped under a predetermined pressure over said flange to prevent their separation;
 2. The invention according to claim 1 wherein said plastic member is an ordnance fuze radome.
 3. a groove circumscribing said outer wall and located so as to allow elastic springback thereof and relieve any load on said flange to ensure that said plastic flange is not permanently stressed by said crimped outer wall whereby said plastic member is free to rotate relative to said metal member.
 3. The invention according to claim 2 wherein said metal member is an ordnance fuze body.
 4. A method of fabricating a stress-free joint between a plastic radome and the metal body of an ordnance fuze, comprising the steps of: a. providing a dovetail flange at the open end of said radome; b. providing in the open end of said metal body a channel for receiving said flange; c. providing on said metal body a metal lip adapted to be crimped over said flange; d. providing on said lip a circumferential groove at the optimum position for bending of said lip during crimping without permanently applying stress to said flange by allowing elastic springback of said lip after crimping; e. positioning said flange in said channel; f. applying a predetermined crimping force to said lip, said force causing said lip to bend inward at said groove, said force being sufficient to cause the bent portion of said lip to apply stress to said flange during the continued application of said force but insufficient to cause said lip to apply stress to said flange after removal of said force. 